1. Field of the Invention
The present invention relates generally to coffee percolators, and more particularly, to a coffee percolator having a brewing chamber which includes a boundary wall surface adapted to be adjusted to maintain a constant thickness of the ground coffee bed when using different amounts of ground coffee for brewing different charges of coffee.
2. Description of the Prior Art
Coffee percolators having a brewing chamber for receiving variable amounts of ground coffee are generally known. For example, a coffee percolator of this type is shown in German Patent No. 26 57 355. In this coffee percolator the brewing chamber is of cylindrical configuration and has a cover provided with a closable ground coffee filler opening and a hot water supply tube. Supported above the conical bottom of the brewing chamber is an annular filter sieve adapted to have a bed of ground coffee formed thereon. Above the filter sieve the cross-sectional area of the brewing chamber is substantially constant and invariable over the full height of the brewing chamber. When using this coffee percolator for brewing variable charges of coffee, the brewing chamber is filled with correspondingly varying amounts of ground coffee. This results in a variable thickness of the bed of ground coffee formed on the filter sieve. If it is intended for instance to brew only a single cup of coffee, the ground coffee bed is relatively thin. This permits the hot water to percolate relatively fast, so that the resulting coffee is of inferior quality. On the other hand, the time it takes for the hot water to percolate through the ground coffee is excessively extended when a greater amount of ground coffee is metered into the brewing chamber for brewing a greater quantity of coffee, for instance a one-liter jug. The problem of the inferior quality of a single-cup charge has been solved in practical use by overdosing the ground coffee, i.e. by metering a greater amount of ground coffee into the brewing chamber than would in fact be necessary with optimum exploitation of the ground coffee. This method is relatively uneconomical, however, when it is desired to brew a greater number of single-cup charges. The extended percolation time, on the other hand, in the case of greater charges, had to be accepted as unavoidable.
Described in DE-OS 25 18 693 is a coffee filter intended to ensure optimum exploitation of the coffee powder also in the case of variable brewing charges. In this coffee filter, the brewing chamber above the filter sieve is divided into a plurality of compartments of different size. The hot water inlet is arranged so as to open into the smallest compartment. For brewing a small charge of coffee, the ground coffee is supplied only to the smallest compartment before adding a correspondingly metered amount of hot water. For brewing a charge of intermediate volume, all of the ground coffee is again supplied to the smallest compartment. When subsequently adding a correspondingly increased volume of hot water, the excess amount of water is supposed to overflow into the median compartment and to carry a corresponding amount of ground coffee along in the process. Even when it is intended to brew the greatest possible charge, all of the ground coffee is still supplied to the smallest compartment, the subsequently added hot water being then supposed to ensure the distribution of the ground coffee over all of the filter surface. It is to be doubted, however, whether this principle does in fact result in a beverage of improved quality. It is rather to be assumed that the amount of ground coffee carried along by the hot water as it overflows into adjacent compartments is insufficient for brewing a coffee of satisfactory quality also in these adjacent compartments. When the major proportion of the initially metered ground coffee remains in the first compartment, it leaves only relatively little space for hot water, so that the coffee obtained from the first compartment will be very strong, its amount will be insufficient, however, for imparting the desired full flavor to the relatively thin coffee obtained from the other compartments.
In the case of espresso machines, it is shown, for instance from CH Patent 588,243, or from CH Patent 559,535, or from WO 07/01570, respectively, to provide a brewing chamber with an adjustable boundary wall surface. However, this construction serves only the purpose of compacting the ground coffee in the manner typical for the preparation of espresso. In both of the espresso machines described, the hot water inlet is provided on the upper side of the brewing chamber, while the finished beverage exits through one of the vertical sidewalls. The opposite vertical sidewall is disposed on a piston and acts as the adjustable boundary wall surface mounted for displacement substantially parallel to the direction of flow of the hot water through the ground coffee. The hot water percolates through the compacted ground coffee substantially in the direction of the longitudinal axis of the cylindrical body formed by the compacted coffee powder. The height of the compacted cylindrical body may be doubled for preparing two cups of espresso at once. In this case the hot water has to percolate through a bed of ground coffee of considerably greater thickness.
As the hot water in the case of the two first-named espresso machines enters the brewing chamber at one or two locations on the periphery thereof, while the beverage exits through a brewing sieve at the location of one of the vertical sidewalls, the exploitation of the coffee powder is still less efficient than in the case of coffee percolators in which the hot water enters substantially parallel to the brewing sieve and percolates through the coffee powder bed in a substantially vertical direction. The last-named espresso machine is not designed for preparing variable brewing charges.
An adjustment to variable brewing charges, particularly to the degree required in the case of filter coffee percolators, cannot be accomplished with a brewing apparatus of this design without having to put up with a degradation of quality or substantially extended brewing times.
Shown in German Utility Model 85 29 010 is a coffee filter with a manually insertable insert for obtaining uniform thickness of the ground coffee bed irrespective of the funnel-shape of the filter casing. This construction is not suitable, however, for variable brewing charges because the water tends to flow over the ground coffee, particularly in the case of small brewing charges. A displaceable boundary wall surface for adjustment to variable brewing charges is not provided.